1. Field of the Invention
This invention relates to an electrodeposition coating system, and more particularly to an electrodeposition coating system comprising a first electrode immersed in an aqueous solution of a substance for forming a coating film and a second electrode provided in association with the first electrode, the second electrode being provided with a membrane portion for separating the second electrode from the first electrodes and the aqueous solution.
2. Description of the Prior Art
The electrodeposition coating is broadly divided into two including one using a coating material of anion type and the other using a coating material of cation type. Since, in either of these electrodeposition coatings, uniformity and adhesion of the coating on an article to be coated are excellent and the degree of causing pollution is low, these electrodeposition coatings have recently been widely applied to the automatic coating film treatment of motor vehicle bodies and so forth for example, particularly as suitable ones in the prime coating or one coat finishing for the coating of metal materials.
Out of the coating materials used in these electrodeposition coatings, as the coating material of anion type, one, in which carboxyle group is adhered to resin having a molecular weight (MW) of 2000 so as to be water-soluble, is used, while, as the aforesaid coating material of cation type, one, in which amino group is attached to a resin component of the coating material so as to be water-soluble, is used. On the other hand, even with these water-soluble coating materials, the degrees of ionization after being dissolved in the water are very low. For this, at present, in the case of the coating material of anion type, an alkaline neutralizing agent such for example as triethylamine is mixed thereinto, while, in the case of the coating material of cation type, an acidic neutralizing agent such as acetic acid is mixed thereinto, whereby, in the both cases, neutralizing is effected, respectively, to thereby increase the degrees of ionization in the water.
The neutralizing agents are mixed for increasing the degrees of ionization in accordance with the properties of the resin components of the respective coating materials as described above. On the other hand, when the electrodeposition treatment of the articles to be coated advances to decrease the resin component in the solution, the coating material should be successively supplied from the outside. Accordingly, in the solution, there is accumulated amine or acetic acid as the neutralizing agent, whereby a phenomenon such as redissolving of the coated surface or occurrence of pin holes is generated, so that the efficiency of the electrodeposition coating is impaired to a considerable extent.
For this, recently, as described in Japanese Patent Kokoku (Post-Exam. Publn.) No. 22231/1970 for example, such a so-called pH control is performed for increasing the efficiency that, one electrode is separated from the article as being the other electrode and an aqueous solution by use of an ion-exchange membrane or the like, and amine or acetic acid is osmotically extracted by use of the ion-exchange membrane or the like, to thereby prevent the neutralizing agent from increasing in the aqueous solution.
The electrodeposition of cation type using a coating material of cation type will hereunder be described.
In the electrodeposition of cation type, there has been used an anion exchange membrane as a membrane. This anion exchange membrane normally has a value of 8-10.times.10.sup.-6 (mol/Coulomb) as an electric efficiency of removing the acid (Coulomb acid removing rate).
The acid (neutralizing agent) added to the aqueous solution (ED bath coating material) in the electrodeposition bath amounts to a value A contained in the coating material supplied to the electrodeposition bath.
On the other hand, the acid taken out from the ED bath coating material to the outside totally amounts to a value B, which includes:
(1) 10%-20% of the value A taken out as the acid contained in a UF filtrate used as a rinsing liquid after the electrodeposition coating. PA1 (2) 5%-10% of the value A taken out as the acid contained coating film. PA1 (3) 70%-80% of the value A, which is removed by the membrane electrodes.
Although it is ideal that the value A is equal to the value B, it is difficult to adjust to obtain such an equality. In general, B&gt;A is adopted, whereby if needed, a small amount of acid is added to the bath to keep exact acid balance.
For this reason, when all of the electrodes provided in the electrodeposition bath are turned to be the membrane electrodes, removal of the acid becomes highly excessive, whereby such disadvantages are presented that the acid as being the neutralizing agent lacks and the acid needs to be periodically supplied from the outside and so forth, so that the control of the neutralizing agent in the ED bath coating material becomes troublesome and the acid is uselessly consumed. For this, nowadays, some of the electrodes are constituted by so-called bare electrodes having no membranes, so that removal of the acid can be well balanced.
As described above, when the rate of removal is 8-10.times.10.sup.-6 (mol/Coulomb), removal of the acid becomes excessive and when the rate of removal is 5-6.times.10.sup.-6 (mol/ Coulomb), removal of the acid becomes ideally balanced, so that a neutral membrane having the latter rate of acid removal may be used sometimes.
In addition, in the electrodeposition of cation type, some of the membrane electrodes may use cation exchange films.
However, the technique of using the bare electrodes as some of the electrodes in the above-described example of the prior art presents the following serious disadvantages in view of meeting the requirement for high quality finishing of coating in recent years.
Namely, a sludge mainly containing an inorganic pigment educed on the surfaces of the bare electrodes is problematical and useful components in the coating material tend to contain a component facilitating the electrolytic corrosion of the electrodes (in general, there are many cases where SUS 316 are used), thereby causing a drastic electrolytic corrosion during current passage. Normally, the rate of electrolytic corrosion of SUS 316 is about 2-3.times.10.sup.-6 (g/Coulomb) to the passing current, however, in the above case, the rate of electrolytic corrosion may reach even 100-150.times.10.sup.-6 (g/Coulomb).
As described above, there have been presented such disadvantages that heavy metal ions (Fe, Cr, Ni, etc.) and the like, which are melted out of the electrodes due to the electrolytic corrosion, are mixed into the coating material, whereby surface roughening of a coated surface, lowered rust prevention, coloring by the heavy metals and so forth are caused.
Furthermore, even in the case of use of the neutralizing film in the above-described example of the prior art, the disadvantages same as above have been presented because the neutralizing film allows the components, the heavy metal ions and the like, which facilitate the electrolytic corrosion, to pass therethrough.
Further, in the case of use of the cation exchange film in the above-described example of the prior art, the Coulomb acid removing rate is as low as 1.times.10.sup.-6 (g/Coulomb) or therebelow, and the cation exchange films are used in some of the all membrane films, so that the acid can be prevented from being excessively removed. However, a passing rate of electrolytic corrosion reaches even 4-15.times.10.sup.-6 (g/Coulomb) due to the passage of the heavy metal ions melted out of the electrodes. In other words, this means the disadvantage that, when the cation exchange films are used, almost all of the heavy metal ions melted out of the electrodes are mixed into the coating material.